Adenosine is a ubiquitous biochemical messenger. Adenosine binds to and activates seven-transmembrane spanning G-protein coupled receptors, eliciting a variety of physiological responses. Adenosine receptors are divided into four known subtypes (i.e., A1, A2a, A2b, and A3). These receptor subtypes mediate different, and sometimes opposing, effects. Activation of the adenosine A1 receptor, for example, elicits an increase in renal vascular resistance, while activation of the adenosine A2a receptor elicits a decrease in renal vascular resistance.
In most mammalian organ systems, periods of metabolic stress result in significant increases in the concentration of adenosine in the tissue. The heart, for instance, produces and releases adenosine to mediate adaptive responses to stress, such as reductions in heart rate and coronary vasodilatation. Likewise, adenosine concentrations in kidneys increase in response to hypoxia, metabolic stress and many nephrotoxic substances. The kidneys also produce adenosine constitutively. The kidneys adjust the amount of constitutively produced adenosine in order to regulate glomerular filtration and electrolyte reabsorption. Regarding control of glomerular filtration, activation of A1 receptors leads to constriction of afferent arterioles, while activation of A2a receptors leads to dilatation of efferent arterioles. Activation of A2a receptors exerts vasodilatory effects on the afferent arteriole. Overall, the effect of activation of these glomerular adenosine receptors is to reduce glomerular filtration rate. In addition, A1 adenosine receptors are located in the proximal tubule and distal tubular sites. Activation of these receptors stimulates sodium reabsorption from the tubular lumen. Accordingly, blocking the effects of adenosine on these receptors produces a rise in glomerular filtration rate and an increase in sodium excretion.